The Section on Intracellular Protein Trafficking investigates the molecular mechanisms that determine the sorting of transmembrane proteins in the endosomal-lysosomal system. Sorting processes such as rapid internalization from the plasma membrane, targeting to lysosomes and delivery to the basolateral plasma membrane of polarized epithelial cells are all mediated by interactions between signals in the cytosolic domains of integral membrane proteins and adaptor proteins associated with the cytosolic face of membranes. Two major families of sorting signals, referred to as tyrosine-based and dileucine-based, have been described. In previous work, we have demonstrated that tyrosine-based signals are recognized by the medium (mu) subunits of four adaptor protein (AP) complexes named AP-1, AP-2, AP-3 and AP-4. We have recently discovered that a subset of dileucine-based signals preceded by a large cluster of acidic amino acids are recognized by the GGAs, a novel family of adaptor proteins associated with the trans-Golgi network (TGN) in an ARF-dependent fashion. Acidic cluster-dileucine signals are found in the cytoplasmic domains of intracellular sorting receptors such as the mannose 6-phosphate receptors (MPRs) that carry lysosomal hydrolases from the TGN to lysosomes, as well as of other transmembrane proteins such as beta-secretase, an enzyme involved in the generation of the beta-amyloid precursor linked to Alzheimer?s disease. In collaboration with James Hurley (NIDDK), we have solved the crystal structure of the VHS domain of the GGAs in complexes with sorting signals from the MPRs. We have also elucidated the structural bases for the regulation of signal-VHS interactions by phosphorylation. Finally, we have identified a novel type of clathrin- and GGA-coated carriers that transport MPRs from the TGN to endosomes. Current work is aimed at determining the roles of AP complexes and GGAs in cell physiology, and investigating the possibility that defects in these proteins underlie some lysosomal disorders.